The diverse nature of AC loads demands careful characterization of the power
source requirements. A statement that simply reference a
MIL-STD-1399/MIL-STD-704 interface requirements is often not sufficient.
AC power requirements including output configuration, voltage, frequency, and
power quality should also be carefully examined.

Output Power RequirementsTo define adequately the output power
requirements, the following parameters, as a minimum are needed because of the
complexity of AC power:

Real and Apparent Power - The differences between these numerical values
exists because voltage and current waveforms in AC circuits are not necessarily
identical in shape and phase. The Real and Apparent powers are the same
only with a purely resistive load. All other AC loads contain either
non-linear or reactive components that increase the apparent
power.

The non-linear component introduces harmonics to the load
current waveshape. These harmonics do not create useful output power, but
increase the RMS value of the output current. These harmonics also
interact with impedances in the distribution system, resulting in distortion of
the voltage waveshape.

Reactive current is introduced by linear reactive
components. Reactive current circulates within the power
distribution system, contributing to losses in, and heating of, cables and
connectors. Where multiple AC output power sources are connected in
parallel, the presence of circulating reactive currents may cause unequal
load sharing.

Crest Factor - The Crest Factor is useful in ascertaining the
ability of the AC Source to power the non-linear rectified capacitor loads,
since these loads draw high peak currents. As a guideline, AC sources
with a Crest Factor capacity above 2.5 are generally adequate to support
end-user equipment.

Power Factor - Power Factor is the ratio
of Real load power to the Apparent load power. Most commonly, this
parameter is used to identify the ability of the AC Source to power reactive
loads. The reactive power capability often complicates the
design. Therefore, in attempts to lower product cost, many
manufacturers do not offer this capability. Ensure that the reactive
power requirement is specified if the load is expected to have a reactive
component.

Non-Unity Power Factor - Power
distribution systems are also sensitive to the effects of non-unity power
factor, because the presence of reactive line current decreases the real
power capability of most generators and alternators. Therefore, it is
common to constrain the allowable range of power factors that an AC source
may present to the prime power source.

5.1.2 AC Bus Frequencies

The standard AC bus frequencies are 50, 60, and 400 Hz. The output
frequency usually carries no impact on the size and weight of the Pulse Width
Modulated Inverters. In contrast, the size and weight of the Fundamental
Frequency Inverter topologies (Squarewave, Resonant, and Harmonic Elimination)
are heavily impacted by the output frequency. A 50 Hz Fundamental
Frequency Inverter will normally weigh up to 7 times more than a 400 Hz inverter
with an identical power rating.

Most end-user equipment is designed to tolerate moderate frequency
variations, but some products are designed to rely on the accuracy of the output
frequency. For critical applications, frequency stability and accuracy
should be specified.